1. Field of the Invention
The present invention relates to a multi-channel colorimeter for measuring the colorimetric property of light to be measured (incident light) such as light emanated from a source or light reflected or transmitted from an illuminated sample.
2. Description of the Related Art
It is required that a colorimeter for measuring the colorimetric property of the incident light is provided with a spectral sensitivity approximate to the color-matching function of the Standard Observer, as recommended by the Commission Internationale de l'Eclariage (CIE). In recent years, as the use of a light source emitting monochromatic light or combined monochromatic light, as represented by displays such as a liquid crystal display or a plasma display, as well as an LED (light emitting diode; in general, semiconductor light emitting element) has been popular, a need is increased for higher precision in approximation to the color-matching function for the purpose of precisely measuring the colorimetric property of light emitted from these light sources.
Conventionally, for realizing a spectral sensitivity approximate to the color-matching function, the following two systems have been used:                (1) a filter system building total filter characteristics approximate to the color-matching function by combination of multiple optical filters;        (2) a spectral system measuring a spectral intensity of incident light at a wavelength interval of 5 nm, for example, and approximating the color-matching function by weighted integration of the measured spectral intensities.        
Among these systems, it is difficult to meet the precision requirements in the approximation with the system (1) due to a limitation on usable filters or a variation in filter characteristics. Therefore, the spectral system (2) is becoming dominant. In the spectral system, spectral means for acquiring a spectral intensity of incident light at each wavelengths is an essential element. A colorimeter often measuring non-constant light primarily uses a polychromater which comprises a wavelength-dispersing element and a sensor array and enables to simultaneously measure at all the wavelengths.
An example of the spectral system is known, as disclosed in Japanese Unexamined Patent Publication No. 2002-13981. The publication recites a multi-channel colorimeter having a relatively large wavelength interval of about several ten nm and a large half bandwidth in which a composite spectral sensitivity approximate to a specific spectral sensitivity is obtained by integrating spectral sensitivities of respective wavelength channels multiplied respectively by predetermined weights. The multi-channel colorimeter has various merits such as:                (a) the approximation precision of the spectral sensitivity is higher than that obtained by the filter system (1);        (b) a large amount of light incident into each wavelength channel and a high S/N ratio are obtainable because of the large half bandwidth; and        (c) a silicon photodiode array having superior linearity, S/N ratio and dynamic range is usable due to relatively small number of wavelength channels, and accordingly, use of a charge transferring sensor array elements such as CCDs (solid-state image sensing elements) having poor performance in these parameters is avoidable.        
The multi-channel colorimeter has the aforementioned various merits. However, the approximation error is not reduced enough for measuring the colorimetric property of monochromatic light from the aforementioned various displays, LEDs, or the like. In the conventional multi-channel colorimeter, the weighting factors are set in such a manner that an approximation error (difference) between a composite spectral sensitivity and a target color-matching function is minimized as a whole by a least-square method or a like technique, specifically, such that the sum of squares of a difference between the composite spectral sensitivity and the color-matching function recommended by the CIE at each wavelengths is minimized. In measuring the colorimetric property of the monochromatic light with use of the conventional multi-channel colorimeter, a composite spectral sensitivity which minimizes the approximation error as a whole is employed, despite the fact that part of the color-matching function, namely, merely the wavelength region where the monochromatic light has a certain intensity contributes to the measurement. Such a measurement may degrade the measurement precision, if the approximation precision in the wavelength region corresponding to the monochromatic light is low, despite the fact that the approximation precision as a whole is high.